1. Field of the Invention
The present invention relates to an electron gun device, and more particularly, to an electron gun device having an improved structure in which a voltage is applied to an electron gun installed on a neck portion of a cathode ray tube.
2. Description of the Related Art
In general, a color cathode ray tube using a triode is shown in FIG. 1. Referring to FIG. 1, the cathode ray tube comprises a panel 11 where a fluorescent film having a predetermined pattern is formed, and a funnel 13 sealingly combined with the panel 11 and including a neck portion 13a containing an electron gun 16 and a conical portion 13b where a deflection yoke 17 is installed. A lead pin 18 for applying a predetermined voltage to each electrode of the electron gun 16 is installed at the end of the neck portion 13a. 
In the cathode ray tube, electron beams emitted from the electron gun 16 are selectively deflected by horizontal and vertical deflection magnetic fields formed by the deflection yoke 17 and excite the fluorescent material of the fluorescent film to form an image. Here, as shown in FIG. 2, when electron beams emitted from the electron gun 16 are deflected horizontally by the deflection yoke 17, a horizontal deflection magnetic field 22 of a pin-cushion type focuses the electron beams vertically, to horizontally elongate a cross section of the electron beams such as indicated by reference numeral 24.
Also, when electron beams are deflected vertically by the deflection yoke 17, a vertical deflection magnetic field 21 of a barrel type diverges the electron beams, to horizontally elongate a cross section to form electron beams such as indicated by reference numeral 25. This horizontally elongated distortion of an electron beam becomes more severe toward the peripheral portions of the panel 11 of FIG. 1, to form a halo effect 26a around a fine-focus portion 26b of the electron beams 26. Accordingly, the form of the electron beams landing on the fluorescent film is irregular and the focus characteristic becomes deteriorated, deteriorating the resolution of an image.
To solve these problems, an electron gun of a conventional dynamic focus type has been developed. A dynamic focus type electron gun includes a plurality of focus electrodes forming a quadruple lens between a final accelerating electrode and a focus electrode neighboring the final accelerating electrode or between the focus electrodes. Accordingly, a dynamic focus voltage synchronized with a deflection signal is applied to one or more focus electrodes forming the quadruple lens, to compensate for the distortion of the electron beam due to the non-uniform magnetic field of the deflection yoke.
One example of the dynamic focus type electron gun is disclosed in U.S. Pat. No. 4,945,284. The electron gun includes two or more focus electrodes electrically connected to a resistor for forming an electron lens, and an AC voltage is applied to one of the focus electrodes from a flyback transformer, i.e., a voltage supply source. Accordingly, the AC voltage reduced by the resistor is applied to other focus electrodes connected to the resistor.
Another example of the conventional dynamic focus type electron gun is disclosed in U.S. Pat. No. 5,519,290, as shown in FIG. 3. A cathode xe2x80x98Kxe2x80x99 being an electron emitter, and first through sixth grids G1 through G6 for forming an electron lens are sequentially installed from the cathode xe2x80x98Kxe2x80x99 as shown in FIG. 3. Each of the grids G1 through G6 include an electron beam passing hole serially arranged, and a vertically elongated electron beam passing hole xe2x80x98H1xe2x80x99 and a horizontally elongated electron beam passing hole xe2x80x98H2xe2x80x99 are formed in facing surfaces of the fifth and sixth grids G5 and G6, respectively. A dynamic focus voltage Vf synchronized with the deflection signal is applied to the third and sixth grids G3 and G6, and the voltage applied to the sixth grid G6 is effectively reduced by an electrostatic capacitance formed between the resistor xe2x80x98Rxe2x80x99 and the fifth and sixth grids G5 and G6, to applying the reduced voltage to the fifth grid G5.
However, problems remain in the above-described dynamic focus type electron gun as follows.
First, since the electrostatic capacitance between the electrodes and the characteristics of the resistor connecting the fifth and sixth grids are liable to change during manufacturing of the electron gun, it is difficult to control the reduction of voltage and the focus characteristics becomes deteriorated. For example, an error in the spacing between the electrodes during the process of manufacturing the electron gun or a change in the electrode surface area during manufacturing of the electrode due to molding process causes a change in the electrostatic capacitance.
Second, the resistor connected to the fifth and sixth grids is in the sealed cathode ray tube. Accordingly, the resistor cannot be replaced and the resistance cannot be changed. For example, a high voltage may be applied to the grid during aging the cathode ray tube. Accordingly, due to a high voltage surge flowing through the resistor, the resistor can be fractured or the characteristics thereof may be spoilt. Also, driving voltages applied to the electron gun may be different according to the electrical appliances employing the electron gun, so that the resistance of the resistor need to be changed.
To solve the above problems, it is an object of the present invention to provide an electron gun device capable of changing the dynamic focus voltage applied to an electrode of the electron gun.
Accordingly, to achieve the above object of the present invention, there is provided an electron gun device including: an electron gun having an electron emitter, and at least two focus electrodes for forming an electron lens for focusing and accelerating electron beams emitted from the electron emitter, a voltage applying unit for applying an AC voltage to at least one of the focus electrodes through a lead pin connected to the focus electrode, and a voltage drop unit for reducing the AC voltage applied from the voltage applying unit to one of the focus electrodes to apply the reduced voltage to the lead pin connected to the other focus electrode.
The voltage applying unit includes: a rectifier for transforming alternating current to direct current; a condenser for removing high frequency noise of the rectified voltage; a voltage divider for dividing the voltage of the condenser; and a dynamic voltage generator for applying an AC voltage synchronized with a voltage applied to a deflection yoke to the divided voltage.
Also, the voltage drop unit includes a resistor reducing the voltage which is divided by the voltage divider and to which the AC voltage is applied by the dynamic voltage generator.
Preferably, the voltage drop unit includes a rheostat.